when Steam allows will change pretty dam quick if Gog gets an early part download of the game + yes Gog could totally do that if Larian allowed it but Larian is unlikly to have sorted that out
back in the day Steam crashed and Gog was so slow as to be painful and that was only the EA
as a tech issue yes its easy enough to allow 70G of the 80-100g full game to start being downloaded... i don't see it happening but its not because of any real problem
